I. Field of the Invention
This invention relates generally to cardiac devices, and more particularly to the assessment of hemodynamic status by a cardiac device.
II. Description of the Related Art
A critical function of implantable cardioverter defibrillators (ICDs) is to identify and terminate hemodynamically unstable arrhythmias such as ventricular fibrillation (VF) and ventricular tachycardia (VT). A technical challenge is to achieve high sensitivity, so that the occurrence of such arrhythmias does not go undetected, while maximizing specificity, so that relatively benign rhythms such as atrial fibrillation (AF) and sinus tachycardia (ST) are not treated.
Conventional implantable cardioverter defibrillators (ICDs) perform discrimination through analysis of the intracardiac electrogram, where features such as rate, interval regularity, and QRS morphology are analyzed in order to identify the underlying rhythm. This approach is convenient because electrical sensing can be easily performed using the same leads that the device requires to deliver therapy. Furthermore, electrogram features provide reasonable accuracy in rhythm identification. However, the accuracy of this approach is ultimately limited because there are inherent ambiguities in cardiac electrical activity that prevent perfect identification. For example, VT, which typically generates a wide QRS complex morphology and regular intervals, may exhibit narrow complexes, particularly if bipolar sensing is used, and irregular intervals. On the other hand, AF, which typically generates narrow QRS complexes and irregular intervals, may exhibit a wide QRS complex, due to a fixed- or rate-related bundle branch block or interventricular conduction defect. In addition AF may produce extended sequences of regular intervals. Thus, there is a theoretical limit to the accuracy with which electrogram analysis can identify cardiac rhythms. The problem is compounded by the fact that practical considerations, such as power consumption and manufacturing costs, impose compromises in the algorithms and sensing configurations that are implemented.
The conventional approach to the detection of pathological arrhythmias using electrogram analysis rests on the premise that the type of therapy is appropriately determined by the underlying cardiac rhythm. In fact, from a clinical perspective, a more important criterion for determining therapy than the identification of the underlying rhythm is whether the rhythm is hemodynamically stable, that is, whether the heart is pumping blood sufficiently to ensure adequate perfusion of vital organs. For example, there are forms of VT that are hemodynamically stable which do not require immediate cardioversion. For these rhythms, low energy methods of termination such as anti-tachycardia pacing can be attempted without sacrificing safety. Such an approach improves device longevity, and avoids subjecting the conscious patient to painful and distressing electrical shocks. Conversely, there are cases of AF and supraventricular tachycardias that are hemodynamically unstable because of high rate or poor ventricular function, and therefore require rapid termination. Thus, the most relevant question clinically is not the identification of the underlying cardiac rhythm but rather whether the rhythm is hemodynamically stable. This point is well illustrated by the Universal Algorithm for emergency cardiac care, advocated by the American Heart Association, and described in the book Advanced Cardiac Life Support, R. O. Cummins, Ed., 1997. The major branch point, which occurs early in the algorithm, is the test for the presence of an arterial pulse, a rapid and robust method of assessing hemodynamic status. Subsequent evaluation and treatment along the two branches of the algorithm varies significantly depending on the outcome of this test. Only later in the algorithm is identification of the origin, ventricular vs. supraventricular, of the cardiac rhythm attempted.
Because of the inherent limitation in the accuracy that can be achieved in identifying cardiac rhythms using electrogram analysis, and more importantly, because what is of ultimate clinical relevance is the hemodynamic status of the patient, what is needed is a method and apparatus for rapidly assessing hemodynamic status that can be used by implantable cardiac devices.
Another application of rapid hemodynamic status assessment is in capture verification, a technique that verifies that a pacemaker-delivered stimulus has electrically captured the myocardium and initiated the propagation of a depolarization. The technique is useful because it allows continuous or periodic adjustment of the pacing energy to accommodate a changing threshold. It allows the pacemaker to deliver the minimum energy necessary to consistently capture the heart, which both maximizes the pacemaker longevity and enhances patient safety. A method of capture verification that is known in the art analyzes the electrical evoked response that is generated after a pace stimulus is delivered by the pacemaker. While this approach, disclosed in U.S. Pat. Nos. 5,165,404 and 5,165,405, has proven to be commercially viable, it requires sophisticated circuitry and low-polarization leads, and is potentially susceptible to electrical noise. It would be advantageous to have a capture verification technique based on hemodynamic status assessment.
One of the challenges of electrically detecting cardiac depolarizations is ensuring that the system is sufficiently sensitive so that myocardial depolarizations are recognized by the device, while at the same time not excessively sensitive so that repolarization waves or noise, such as that induced by diaphragmatic myopotentials, is incorrectly interpreted as a depolarization. Beat-to-beat hemodynamic sensing would allow oversensing of electrical noise to be recognized and distinguished from ventricular fibrillation (VF) or tachycardia. In oversensing of electrical noise, regular and robust mechanical cardiac contractions would occur along with higher-rate ventricular sensed events. In this case the sensitivity could be decreased until the ventricular sensed events occurred in concert with cardiac contractions. On the other hand, the detection of robust mechanical cardiac contractions during the absence of ventricular sensed events would indicate that the sense amplifier sensitivity is set too low and should be increased. Finally, the presence of rapid ventricular sensed events without detected cardiac contractions would indicate the presence of VF, hemodynamically unstable VT, or another hemodynamically unstable rhythm. It would thus be advantageous to provide an improved system for verification of sensed events and optimization of sensing thresholds.
Yet another application of hemodynamic sensing is in pace-parameter optimization, in which any of a number of parameters that define pacing characteristics is optimized. Pace-parameter optimization is particularly applicable to multi-site pacing. For example, in dual-chamber (atrial and ventricular) pacemakers the atrio-ventricular (AV) delay is optimized, so that the ventricular contraction is timed such that the contribution of the atrial contraction is maximally exploited. Another example is biventricular pacing for heart failure, in which ventricular synchrony is optimized by adjusting the timing that pace pulses are delivered to various sites. Currently, these parameters are set to default nominal values, or labor-intensive methods are used to assess hemodynamics in order to optimize the parameters at time of device implant. Examples of these methods include ultrasound to measure ejection fraction and left heart catheterization to measure the rate of change of left ventricular pressure during systole, which is a measure of contractility and mechanical efficiency. In addition to the substantial time and effort these approaches impose, the invasive techniques increase the perioperative risk to the patient. Furthermore, these approaches are possible only during device implant or follow-up examinations. A hemodynamic sensor incorporated into the device would allow frequent and dynamic optimization of pacing parameters.
In pace-parameter optimization, some measure of cardiac function is needed to serve as the feedback which determines how the pace parameters are adjusted. The measure of cardiac function provides the basis of optimization; it allows one set of parameter values to be deemed superior to another. To be fully general the measure must operate over short time scales, ideally on a beat-to-beat basis. While a slower response may be adequate for adjusting, for example, the overall pacing rate, a rapid response is necessary to dynamically adjust multi-site timing to changes in body posture, which is particularly important for certain diseases such as hypertrophic obstructive cardiomyopathy and dilated cardiomyopathy. Furthermore, a sensor capable of detecting changes in hemodynamic parameters on a beat-to-beat basis would allow the optimization algorithm to achieve the ideal parameter values quickly.
Still another application that would benefit from hemodynamic sensing is in the monitoring of disease status in heart failure patients. Hemodynamic sensing would allow the optimization of medical management of the heart failure patient. It would also allow the early recognition of a developing acute exacerbation. With early recognition the exacerbation could be relatively easily terminated, before it becomes fully developed and requires hospitalization and intensive intervention to terminate.
A further application of hemodynamic sensing is in the assessment of autonomic status. For example, the development of diabetic neuropathy is known to adversely affect the sympathetic/parasympathetic balance and the body""s response to autonomic perturbations. Since the control of cardiovascular system is in large part the responsibility of the autonomic nervous system, sensitive hemodynamic measurements can detect changes in autonomic tone, such as parasympathetic decline associated with the development of diabetic neuropathy or the sympathetic enhancement provoked by a CHF exacerbation.
Techniques for hemodynamic assessment are known in the art. In the context of arrhythmia discrimination Cohen and Liem (Circ., 1990, 82:394-406) systematically studied the use of a pressure transducer placed in the right ventricle, a configuration described in U.S. Pat. No. 4,774,950, and demonstrated that this approach effectively discriminated hemodynamically stable from hemodynamically unstable cardiac rhythms. The approach of Cohen and Liem is advantageous in that it directly characterizes important hemodynamic variables, namely central venous pressure and the pressure differentials that the heart is able to generate. However, incorporation of a hemodynamic sensor into an intracardiac lead is undesirable for several reasons. First, it increases the cost of the lead, which because of the significant engineering challenges associated with providing chronic intracardiac placement, already represents a sizable fraction of the overall cost of the system. More importantly, incorporation of a special sensor in an otherwise general-purpose lead imposes expensive compatibility constraints: any device that would make use of the sensor must have the special-purpose lead implanted with it, and once the lead has been implanted, any subsequent device changes, such as for upgrade or battery depletion, must have hardware capable of interfacing with the special-purpose sensor. Finally, a special class of patients, such as some pediatric patients and patients with congenital cardiovascular malformations, require epicardial placement of leads; intracardiac placement is not possible for them. These patients would not benefit from a hemodynamic sensing method that requires intracardiac placement. It is therefore desirable to provide a method and apparatus for hemodynamic sensing that does not require special intracardiac or intravascular leads. Such an invention would avoid expensive, special purpose leads. Furthermore, it would avoid imposing expensive lead-device compatibility constraints.
Erickson and Bennett, in U.S. Pat. No. 5,176,137, propose placing a two-wavelength oxygen saturation sensor in the right ventricle of the heart. Their disclosure assumes that during hemodynamically stable rhythms the oxygen saturation in blood returning to the right ventricle is pulsatile, and that the pulsatile character of the saturation decreases during hemodynamically unstable rhythms. Their disclosure further assumes that the average saturation level of the blood returning to the right ventricle decreases during hemodynamically stable sinus and ventricular tachycardias but remains approximately constant during unstable rhythms. The intracardiac placement of their sensor carries the disadvantages described above. More importantly, while slow changes in venous oxygen saturation can be expected with certain rhythms, the amount of time required for blood to return to the right ventricle from the periphery is substantial, on the order of tens of seconds. This delay is even longer for hemodynamically unstable rhythms. Delaying ICD therapy by this amount of time would compromise patient safety. What is needed is a method of assessing hemodynamic status that is robust and rapid. Ideally the method operates on the time scale of a single cardiac cycle, i.e., on a beat-to-beat basis.
Other examples of oxygen sensors exist in the art. U.S. Pat. Nos. 4,399,820, 4,467,807, 4,815,469, and 5,040,538 all present oxygen saturation or partial pressure sensors placed in the right ventricle for rate responsive pacing, in which the pacing rate of the pacemaker is controlled based on the metabolic demand of the body, which is a form of hemodynamic assessment and pace-parameter optimization. Assuming arterial O2 is constant, a fall in venous O2 below a critical level implies that the cardiac output is not sufficient to meet metabolic demand. In this case, a pacing parameter, the pacing rate, is increased. The time scale of this process is on the order of tens of seconds, much greater than the beat-to-beat assessment that is necessary to perform arrhythmia discrimination, capture verification, sensing gain and threshold optimization, and rapid pace-parameter optimization. Furthermore, the intravascular placement carries the disadvantages discussed above.
In U.S. Pat. No. 5,540,727, Tockman et al. disclose an algorithm for optimizing pacing parameters, including pacing mode. They mention a number of examples of a variety of measures of hemodynamic status, including both implantable embodiments (cardiac output measured using impedance plethysmography of the right ventricular volume, and right ventricular pressure) and external embodiments (cardiac output measured using Doppler ultrasound, heart sounds, blood pressure, respiratory gas analysis, and pulse oximetry). External measurements of hemodynamic status are labor-intensive and can only be used during periodic follow-up examination. They are therefore not suitable for arrhythmia discrimination, dynamic pace-parameter optimization, sensitivity optimization, or capture verification. The measures they present for implantable embodiments all require right ventricular placement, and thus carry with them the disadvantages described above.
In U.S. Pat. No. 5,334,222, Salo et al. present a system tailored to heart failure patients that includes dual chamber (atrial and ventricular) pacing with defibrillation capability. An algorithm for optimizing AV delay is disclosed which uses information generated by a generic sensor of cardiac function. Particular embodiments of the sensor that are described include intracardiac impedance plethysmography to assess stroke volume and cardiac output, an intracardiac pressure transducer, and Doppler ultrasound to assess blood flow velocity. An additional modality that is described is heart sounds, though neither a physical embodiment of a heart sound sensor nor a method of using heart sound to obtain a measure of cardiac function are disclosed.
Methods of pace-parameter optimization have been proposed using non-hemodynamic surrogates, such as QRS width. In U.S. Pat. No. 5,527,347 Shelton and Warkentin adjust the AV delay such that QRS width is maximized. Their invention is tailored to patients with hypertrophic obstructive cardiomyopathy, in whom a thickened intraventricular septum obstructs the ejection of blood from the left ventricle during systole. Delivering a pre-excitation pulse to the right ventricular apex is thought to pull the septum toward the right side of the heart, thereby reducing the degree of obstruction. In the patent, maximizing the QRS width is claimed to provide the optimal AV delay.
In contrast to maximizing QRS width, minimizing the QRS width has been proposed in biventricular pacing for patients with dilated cardiomyopathy. In these patients, improving the synchrony of ventricular contractions is thought to improve cardiac function to a clinically significant degree. QRS width serves as a marker for the degree of synchrony. However, recent results of Kass et al. (Circ. 1999; 99:1567-1573) showed that in heart failure patients QRS width tends to increase in VDD pacing, regardless of the pacing site and despite the significantly improved hemodynamic performance in some of the pacing configurations they tested. Thus the suitability of QRS width as a hemodynamic surrogate is suspect in this patient population.
Use of the QRS width as a feedback variable in pace-parameter optimization schemes such as those just described is convenient because the intracardiac or far-field electrogram is readily available. However, while the rationale for using QRS width in special settings of pace-parameter optimization is intuitively appealing, QRS width is not itself a measure of hemodynamic function, which is what would ideally be optimized in pace-parameter adjustment. Furthermore, while QRS width can provide useful information in the special cases just described, it is not generally applicable. For example, for pacemaker-dependent patients with complete heart block, adjusting the AV delay does not affect the QRS width in any meaningful way. Assessment of hemodynamic performance on a beat-to-beat basis would be a direct and general way to provide for the optimization of pace parameters. It would be operable both in the common applications of sinus node dysfunction and heart block, as well as the special cases of biventricular pacing in heart failure and dual chamber pacing in hypertrophic obstructive cardiomyopathy. Indeed, the information it provides would be interpreted the same way whether the goal of therapy is to improve ventricular synchrony, as is the case with dilated cardiomyopathy, or to decrease synchrony, as with hypertrophic obstructive cardiomyopathy.
In U.S. Pat. No. 5,554,177, Kieval and Soykan present a pace-parameter optimization system that uses heart sounds as the feedback variable. They note that an acoustic sensor does not necessarily require intracardiac placement, but instead can be placed in the housing of the implanted device. As discussed above, this is advantageous because it does not require costly lead modifications, nor does it impose restrictive device/lead compatibility constraints. In their disclosure, pace-parameter optimization is performed using as feedback a particular heart sound which is xe2x80x9can artifact indication of a less than optimal heart condition.xe2x80x9d Their disclosure isolates the sound in time and adjusts the pacing parameters in order to xe2x80x9clower the volumexe2x80x9d of the sound. In the preferred embodiment the volume of the sound of mitral regurgitation is minimized. The generality of this approach is limited in several ways. Conditions that lack an artifact indication can not support this approach to parameter optimization. Furthermore, not all patients with a given disease, such as the example of dilated cardiomyopathy used by the authors to illustrate the invention, will have a given artifact indication, such as mitral regurgitation. More importantly, this approach, in which a particular abnormal heart sound is minimized, would fail dramatically in some applications of hemodynamic assessment, such as arrhythmia discrimination. Indeed, because it seeks to minimize heart sounds, the measure proposed in the ""177 Patent would falsely classify a hemodynamically unstable rhythm, such as ventricular fibrillation, as hemodynamically stable. Even in application to the examples presented in the ""177 Patent, hypertrophic and dilated cardiomyopathy, one is struck by the technical challenge of determining whether the minimization of the artifact heart sound results from, on the one hand, desirable parameter settings that increase cardiac hemodynamic performance, or, on the other hand, undesirable parameter settings that significantly compromise cardiac synchrony and therefore reduce the degree of regurgitation. The fundamental drawback of the approach the authors describe is that it does not provide hemodynamic sensing. What is reflected in the intensity of the sound of mitral regurgitation is the backward jet of blood into the left atrium from the left ventricle, not the forward output of blood from the heart. What is needed is a measure that is directly related to the hemodynamic function of the heart.
In U.S. Pat. No. 4,356,827, Uemura et al. describe the use of Korotkoff sounds in the detection of cardiac arrhythmias. Korotkoff sounds are the sounds generated when blood is pumped through an artery subjected to an external pressure which lies between the systolic and diastolic pressures. The invention clearly provides a measure of hemodynamic status in that it identifies the systolic and diastolic pressures as being respectively above and below the externally applied pressure. As a measure of hemodynamic status it is generally applicable to the problems presented above. However, the external pressure cuff precludes the use of the invention in an implanted device. A modified embodiment configured for internal use can be imagined, but such an approach would require surgical dissection of an artery and attachment of a pressure cuff, or similar element. The pressure applied by the cuff could be dynamically changed when the arterial pressure is checked, for example, as when tachycardia is detected. However, this approach would clearly increase both the device cost and the difficulty and duration of the implant procedure. These considerations make the practicality of such an approach questionable.
Lekholm, in U.S. Pat. No. 4,763,646, describes identifying various portions of the cardiac cycle using heart sounds to provide timing information for a pacemaker. For example, heart sounds are used to provide detection of ventricular contraction in order to inhibit the delivery of a ventricular pace pulse in a DDD pacer. In addition, an application to arrhythmia discrimination is presented. Specifically, heart sounds are used to determine the heart rate, from which tachycardia is detected. In contrast, changes in amplitude and the frequency spectrum of the heart sounds are used for the detection of fibrillation. This approach in effect detects the reduced hemodynamic performance of the heart that results from ventricular fibrillation (VF). However, because of its rapidly lethal character, it is necessary to quickly terminate VF, regardless of the hemodynamic status of the patient early in the arrhythmia. In contrast to VF, it is in the region of rate overlap between ventricular and supraventricular tachycardias that a transducer which is sensitive to diminished hemodynamic status would be most useful. In this region, however, the invention presented in the ""646 Patent uses heart sounds only to calculate rate, not to assess the hemodynamic performance of the heart. A final application of heart sounds in the ""646 Patent is to capture verification. Other systems using heart sounds are described in U.S. Pat. Nos. 3,985,121, 5,685,317, and 5,687,738, though none of these make use of sounds in hemodynamic assessment.
Regarding the physical location of a sensor used by an implantable device, as described above in the context of pressure sensing in the right ventricle, it is desirable to place the sensor outside the bloodstream. Incorporation of the sensor inside or on the device would be most convenient. Prutchi and Paul, in U.S. Pat. No. 5,556,421 propose placement of a sensor within the header of a cardiac device. A disadvantage of this approach is that it does not necessarily provide for the optimal signal transduction of a particular sensor. For example, the performance of the optical sensor described in the Prutchi and Paul Patent would be so severely degraded by direct transmission of light from source to detector that one skilled in the art would question the functionality of the proposed solution. In addition, placement in a rigid epoxy header is simply not an option for some sensors, such as sound sensors, because of the dramatic degradation in the signal-to-noise ratio that the rigid header would impose. What is needed is a method of incorporating a hemodynamic sensor into a implantable device, providing it optimal access to the external milieu so that the signal of interest is optimally transduced, maintaining the hermetic enclosure provided by the device housing, and minimizing the added volume that the sensor imposes.
Fearnot in U.S. Pat. No. 5,040,533 teaches placement of a generalized window in the housing of the cardiac device. The window might be transparent to facilitate the transmission of light or flexible to facilitate pressure transduction. While the convenience, from the clinician""s perspective, of incorporating the sensors into the housing of the cardiac device is an obvious advantage, the technical difficulty in maintaining a hermetic seal between two different materials, particularly in a chronically implanted device, is equally obvious to one skilled in the art. The technical challenge is made more difficult by the greatly increased circumference, relative to that of standard feed-through connections known in the art, of the boundary between the window and the device housing. What is needed, therefore, is a method of placing a hemodynamic sensor in or on the device without compromising the integrity of the hermetic enclosure.
Because of the considerations described above, the principal object of the present invention is to provide an extravascular sensor that monitors a patient""s hemodynamic status.
An additional goal of the present invention is to provide hemodynamic sensing over short time scales, i.e., on the time scale of a single cardiac beat.
Another object of the invention is to provide a means of determining whether electrical therapy is needed based on the hemodynamic status of the cardiac rhythm.
A further object is to provide a means of capture verification for cardiac pacemakers.
Still another object is to allow the optimization of the sensitivity of sensing circuitry.
Yet another object is to provide immunity from electrical noise in the sensing of cardiac activity.
An additional object of the invention is to provide hemodynamic sensing for the optimization of pacing parameters.
Still another object of the invention is to provide for extravascular placement of the device and sensors, which minimizes the perioperative and long-term risk of complications.
Further objects and advantages will become apparent from a consideration of the ensuing description and drawings.
The present invention is directed to an implantable medical device such as a pacemaker or implantable cardioverter defibrillator or stand-alone hemodynamic monitor that uses an acoustic transducer responsive to heart sounds to detect the hemodynamic status of a patient. A hermetic housing such as a titanium xe2x80x9ccanxe2x80x9d encloses the device electronics and the device housing includes at least one substantially planar face configured to act as a diaphragm in response to acoustic waves. A transducer positioned inside the device housing provides an output signal to the device electronics responsive to vibration of the diaphragm.
In a first embodiment of the invention, the transducer is a piezoelectric element positioned on the diaphragm. In a second embodiment, the transducer comprises a laser interferometery source and detector having a laser light reflecting off of the diaphragm.
The sensed heart sounds signals may be used to discriminate among possible cardiac arrhythmias, particularly hemodynamically stable and unstable arrhythmias such as may be the case between an atrial tachycardia and a ventricular tachycardia.
The medical device of the invention may further include an accelerometer for detecting motion artifacts that could interfere with the sensed heart sounds signals.
In another embodiment the medical device of the invention is implemented in an implantable pulse generator that provides pacing therapy to a patient""s heart. The electronic circuitry uses the sensed heart sounds signals to optimize the timing of pacing pulses provided to said patient""s heart. This may include a posture sensor that triggers optimization of the timing of pacing pulses upon sensing of a change of the patient""s posture. The voltage level of pacing pulses provided to said patient""s heart may also be optimized or pacing capture may be verified. In addition, the sensed heart sounds signals may be used to optimize electrical sensing of cardiac signals.